Method and device for measuring the tilt of an optical disc

ABSTRACT

A method and device for measuring the tilt in an optical disc drive ( 1 ) is disclosed. The optical disc drive ( 1 ) comprises two lasers ( 31, 41 ) generating two laser beams ( 32, 42 ) having mutually different optical characteristics. One of these laser beams ( 32 ) is continuously ON, and is used for writing or reading data to or form the disc. The other laser beam ( 42 ) is repeatedly switched ON and OFF. Tilt is measured by comparing a normalized error signal (RED(ON)) during the ON-phase (TON) with a normalized error signal (RED(OFF)) during the OFF-phase.

FIELD OF THE INVENTION

The present invention relates in general to a disc drive apparatus forwriting/reading information into/from an optical storage disc, whereinthe disc is rotated and a write/read head is moved radially with respectto the rotating disc. The present invention is applicable in the case ofoptical as well as magneto-optical disc systems. Hereinafter, thewording “optical disc drive” will be used, but it is to be understoodthat this wording is intended to also cover magneto-optical discsystems.

BACKGROUND OF THE INVENTION

As is commonly known, an optical storage disc comprises at least onetrack, either in the form of a continuous spiral or in the form ofmultiple concentric circles, of storage space where information may bestored in the form of a data pattern. Optical discs may be read-onlytype, where information is recorded during manufacturing, whichinformation can only be read by a user. The optical storage disc mayalso be a writable type, where information may be stored by a user.

For writing information in the storage space of the optical storagedisc, or for reading information from the disc, an optical disc drivecomprises, on the one hand, rotating means for receiving and rotating anoptical disc, and on the other hand optical means for generating anoptical beam, typically a laser beam, and for scanning the storage trackwith said laser beam. Since the technology of optical discs in general,the way in which information can be stored in an optical disc, and theway in which optical data can be read from an optical disc, is commonlyknown, it is not necessary here to describe this technology in moredetail.

For rotating the optical disc, an optical disc drive typically comprisesa motor, which drives a hub engaging a central portion of the opticaldisc. Usually, the motor is implemented as a spindle motor, and themotor-driven hub may be arranged directly on the spindle axle of themotor.

For optically scanning the rotating disc, an optical disc drivecomprises a light beam generator device (typically a laser diode), anobjective lens for focussing the light beam in a focal spot on the disc,and an optical detector for receiving the reflected light reflected fromthe disc and for generating an electrical detector output signal.

During operation, the light beam should remain focussed on the disc. Tothis end, the objective lens is arranged axially displaceable, and theoptical disc drive comprises focal actuator means for controlling theaxial position of the objective lens. Further, the focal spot shouldremain aligned with a track or should be capable of being positionedwith respect to a new track. To this end, at least the objective lens ismounted radially displaceable, and the optical disc drive comprisesradial actuator means for controlling the radial position of theobjective lens.

In many disc drives, the orientation of the objective lens is fixed,i.e. its axis is directed parallel to the rotation axis of the disc. Insome disc drives, the objective lens is pivotably mounted, such that itsaxis can make an angle with the rotation axis of the disc.

For any reason, the optical disc may suffer from tilt. Tilt of theoptical disc can be defined as a situation where the storage layer ofthe optical disc, at the location of the focal spot, is not exactlyperpendicular to the optical axis. The tilt can have a radial componentand a tangential component. As illustrated in FIG. 6, the radialcomponent (radial tilt) is the angular component β of the deviation in aplane oriented transversely to the track to be read (i.e. along theradial direction R) and transversely to the data carrier, while thetangential component (tangential tilt) is defined as the angularcomponent α of the deviation in a plane oriented parallel to the track(i.e. along the tangential direction T) to be read and transversely tothe data carrier. Tilt can be caused by the optical disc being tilted asa whole, but is usually caused by the optical disc being warped, and asa consequence the amount of tilt depends on the location on disc.Especially systems, which have a relatively large numerical aperture(NA), are sensitive to disc tilt. Therefore, tilt compensationmechanisms have been developed. Typically, in a disc drive apparatushaving tilt compensation, at least the objective lens is mountedpivotably, and the optical disc drive comprises tilt actuator means forcontrolling the tilt position of the objective lens so that the laserbeam remain locally perpendicular to the disc surface. Alternatively, itis possible that the orientation of the disc itself is corrected.

For attenuating the effect of the disc tilt, there is thus a need fordefining a method of measuring the optical disc tilt.

It is possible to measure the tilt with a separate tilt sensor. However,such solution would involve additional hardware and increased costs.

It has already been proposed in prior art to process an electricaloutput signal from the optical detector in order to obtain a tiltmeasuring signal indicating the tilt angle. Based on such a tiltmeasuring signal, a tilt controller can control the tilt actuator meansin such a way that the tilt angle is reduced or even made zero.

The Japanese patent JP-2000 076 679-A discloses a combi-drive intendedto read and write data on optical discs having different formats inusing a plurality of different light beams. A first light beam, referredto as data beam, is used for the writing/reading operation. A secondlight beam, referred to as tilt beam, is used for tilt measurement. Thiscombi-drive comprises means for measuring the tilt. For this purpose,the tilt beam is modulated with a predetermined modulation frequency,resulting in an electrical tilt-indicative signal component having thesame frequency. A band-pass filter is thus used to derive this signalcomponent, which is further processed for measuring tilt. This techniquehas some disadvantages.

On the one hand, this technique requires the use of at least a beammodulator, a band-pass filter, and a peak detector, which adds to thecomplexity and cost of the apparatus.

On the other hand, the tilt beam causes an electrical signal havingmodulation as well as a continue component, which may affect errordetection based on the read signal output. The amount of influence isnot constant but depends on tilt direction and magnitude, therefore therequired compensation of this effect is difficult.

Further, the continuous use of the tilt beam adds to the powerdissipation of the apparatus.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved tiltmeasuring method and device.

To this end, the method according to the invention of measuring the tiltof an optical disc in an optical disc drive comprises:

-   -   a step of directing to the optical disc during a normal phase, a        first laser beam having a first optical characteristic for        writing/reading information into/from the optical disc,    -   a step of deriving a first intermediate value from a first        normalized error signal obtained after reflection of said first        laser beam on the optical disc,    -   a step of directing to the optical disc during a tilt-measuring        phase, said first laser beam and a second laser beam having a        second optical characteristic,    -   a step of deriving a second intermediate value from a second        normalized error signal obtained after reflection of said first        and second laser beams on the optical disc,    -   a calculation step of deriving a tilt-indicative signal from the        difference between said second and first intermediate values.

The principle of the present invention is to use a first light beam,referred to and used as data beam, and a second light beam, referred toand used as tilt beam, having different optical characteristics, whichresults in different tilt sensitivities measured by a photo detector.The optical characteristics, which influence the tilt sensitivity, arefor instance the wavelength, focus, spherical aberration, polarization.In using different laser beams, any disc tilt results in a detectabledifference between on the one hand a tilt-induced deflection of thedetector spot of the first light beam and on the other hand atilt-induced deflection of the detector spot of the second light beam.The second light beam is alternatively switched ON and OFF. Tiltmeasurements are performed during the time periods during which the tiltbeam is ON.

The invention also relates to an optical disc drive apparatus where afirst light beam is used for the writing/reading operation while asecond light beam is used for tilt measurement. This apparatus comprisesmeans for implementing the steps of the above-mentioned method accordingto the invention.

In particular, this optical disc drive apparatus corresponds to acombi-drive capable of handling two or more different disc types, suchas for instance CD, DVD, Blu-Ray, with two or more different laserbeams.

This optical disc drive apparatus may also correspond to a drive capableof handling only one disc types, such as for instance CD or DVD,although this would require the installation of an additional opticalsystem for generating the second laser beam used for tilt measurement.

In a preferred embodiment of the invention, the duration of thetilt-measurement phases is chosen such as to be shorter than a relevanttime scale of expected changes of the error signals intended to correctthe three dimensional position of the objective lens. As a consequence,any possible influence of the tilt beam on the error signal does notaffect the control of the actuators. Moreover, since the tilt beam isnot used continuously, the power dissipation of the apparatus islimited.

In a preferred embodiment of the invention, at least one error signal isignored during said tilt-measurement phases, so that at least one lensactuators is frozen during said tilt-measurement phases.

It is an extra feature and precaution to ensure that the control of theactuators is not affected during the tilt measurement, whatever theduration of the tilt measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which same reference numerals indicatesame or similar parts, and in which:

FIG. 1A schematically illustrates an optical disc drive,

FIG. 1B is a block diagram illustrating schematically an opticaldetector connected to a signal processor,

FIG. 2 is a graph illustrating an error signal as a function of radiallens position,

FIG. 3 is a timing diagram illustrating the operation of a disc drive inaccordance with the present invention,

FIG. 4 is a block diagram schematically illustrating components of apreferred embodiment of a control circuit,

FIG. 5 illustrates the displacement of light beams on a four-quadrantsdetector,

FIG. 6 illustrates a radial tilt and a tangential tilt in an opticaldata carrier.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A schematically illustrates an optical disc drive apparatus 1,suitable for storing information on or reading information from anoptical disc 2, typically a DVD or a CD. For rotating the disc 2, thedisc drive apparatus 1 comprises a motor 4 fixed to a frame (not shownfor sake of simplicity), defining a rotation axis 5. For receiving andholding the disc 2, the disc drive apparatus 1 may comprise a turntableor clamping hub 6, which in the case of a spindle motor 4 is mounted onthe spindle axle 7 of the motor 4.

The disc drive apparatus 1 further comprises an optical system 30 forscanning tracks (not shown) of the disc 2 by an optical beam. Morespecifically, in the exemplary arrangement illustrated in FIG. 1A, theoptical system 30 comprises a first light beam generating means 31 and asecond light beam generating means 41, each typically a laser such as alaser diode, each arranged to generate a first light beam 32 and asecond light beam 42, respectively. In the following, different sectionsof the optical path of a light beam 32, 42 will be indicated by acharacter a, b, c, etc added to the reference numeral 32, 42,respectively.

The first light beam 32 passes a first beam splitter 43, a second beamsplitter 33 and an objective lens 34 to reach (beam 32 b) the disc 2.The beam splitters are schematically depicted as cubes, but may haveother implementations. The first light beam 32 b reflects from the disc2 (reflected first light beam 32 c) and passes the objective lens 34 andthe second beam splitter 33 (beam 32 d) to reach an optical detector 35.

The second light beam 42 is reflected by a mirror 44, passes the firstbeam splitter 43, and then follows an optical path comparable with theoptical path of the first light beam 32, indicated by reference numerals42 b, 42 c, 42 d.

The objective lens 34 is designed to focus one of the two light beams 32b, 42 b in a focal spot F on a recording layer (not shown for sake ofsimplicity) of the disc 2, which spot F normally is circular.

In the following, the first beam 32 will be referred to as data beamwhile the second beam 42 will be referred to as tilt beam.

It is noted that in any optical disc drive, the objective lens isdesigned to form, in combination with the material of the optical disc,an optical system which is optimally adapted to a data beam of a certainwavelength (indicated hereinafter as design wavelength), such that, if alight beam having the design wavelength is being used, and if this lightbeam is substantially parallel (i.e. non-convergent and non-divergent)when entering the objective lens, and if the focus point of this beamcoincides with the storage layer of the disc, the reflected beam issubstantially free from aberration. This condition will be indicated asthe design operation condition, which will be considered as a propertyof the optical disc drive apparatus. If a second beam is used which doesnot meet all design operation conditions, the second reflected beam willbe subject to some distortion like aberration. As a consequence, in atilt situation, the detector spot of such second beam, i.e. the lightspot caused by such second beam when incident on the detector, isdistorted and/or shifted by a different amount than the detector spot ofthe data beam. In the context of the invention, it is thus sufficient ifone of the optical characteristics of the tilt beam differs sufficientlyfrom the corresponding optical characteristic of the data beam.

According to a first example, the tilt beam has a wavelength differingfrom the wavelength of the data beam. In such a case, the data beam andthe tilt beam may be focussed to the same focus spot on the record layerof the optical disc.

According to a second example, the focus point of the data beam and thefocus point of the tilt beam have different locations on the opticalaxis, i.e. these focus points have axial distance with respect to eachother, such that the tilt beam is out of focus when the data beam is infocus on the record layer of the optical disc. In such a case, the databeam and the tilt beam may have the same wavelength.

According to a third example, the polarization condition of the tiltbeam differs from the polarization condition of the data beam, in whichcase the data beam and the tilt beam may have equal wavelength and equalfocus point if the objective lens has refractive properties which arepolarization-dependent or if the optical disc has refractive propertieswhich are polarization-dependent, or both.

According to a fourth example, the wavelength of the tilt beam differsfrom the wavelength of the data beam, while also the focal point of thetilt beam is located at an axial distance from the focal point of thetilt beam.

In the following, for sake of explanation, it will only be consideredthe case where the first laser 31 and the second laser 41 are differenttypes of laser in that their respective laser beams 32 and 42 have adifferent wavelength. It is noted that the explanations would be thesame with a first and second laser beams having different opticalcharacteristics. For instance, in a combi-drive, a laser beam suitablefor handling a CD has a wavelength in the order of about 780 nm, while alaser beam suitable for handling a DVD has a wavelength in the order ofabout 660 nm.

When a combi-drive is in a CD-playing mode, the tilt beam will be theDVD beam, hence the tilt beam has a shorter wavelength than the databeam. When, on the other hand, the combi-drive is in a DVD-playing mode,the tilt beam will be the CD beam, hence the tilt beam has a longerwavelength than the data beam.

In such conditions, the tilt beam is defocused and/or sphericallyaberrated when the data beam is in focus.

The disc drive apparatus 1 further comprises an actuator system 50,which comprises a radial actuator 51 for radially displacing theobjective lens 34 with respect to the disc 2. Since radial actuators areknown per se, while the present invention does not relate to the designand functioning of such an actuator, it is not necessary here to discussthe design and functioning of a radial actuator in great detail.

For achieving and maintaining a correct focusing, exactly on the desiredlocation of the disc 2, said objective lens 34 is mounted axiallydisplaceable, while further the actuator system 50 also comprises afocal actuator 52 arranged for axially displacing the objective lens 34with respect to the disc 2. Since axial actuators are known per se,while further the design and operation of such axial actuator is nosubject of the present invention, it is not necessary here to discussthe design and operation of such focal actuator in great detail.

For the purpose of tilt compensation, said objective lens is mountedsuch as to be pivotable about a joint (not shown) which preferablycoincides with the optical centre of the objective lens 34. Further, theactuator system 50 also comprises a pivot actuator 53, also indicated astilt actuator, arranged for pivoting the objective lens 34 with respectto the disc 2. The pivot actuator 53 is intended to correct the effectof the radial and tangential tilt from control signals S_(CTA) andS_(CTB), said control signals being derived from radial and tangentialtilt angle calculations, respectively, measured according to the presentinvention.

It is noted that means for supporting the objective lens with respect toan apparatus frame, and means for axially and radially displacing theobjective lens, are generally known per se. Since the design andoperation of such supporting and displacing means are no subject of thepresent invention, it is not necessary here to discuss their design andoperation in great detail. The same applies to means for pivoting theobjective lens.

It is further noted that the radial actuator 51, focal actuator 52, andpivot actuator 53 may be implemented as one integrated 3D-actuator.

The disc drive apparatus 1 further comprises a control circuit 90 havingan output 92 connected to a control input of the motor 4, having anoutput 93 coupled to a control input of the radial actuator 51, havingan output 94 coupled to a control input of the focal actuator 52, havinga double output 95 coupled to a control input of the pivot actuator 53,and having an output 96 coupled to a control input of the second laser41. The control circuit 90 is designed:

-   -   to generate at its output 92, a control signal S_(CM) for        controlling the motor 4,    -   to generate at its control output 93, a control signal S_(CR)        for controlling the radial actuator 51,    -   to generate at its output 94, a control signal S_(CF) for        controlling the focal actuator 52,    -   to generate at its double output 95, a control signal S_(CTA)        for controlling the radial position of the pivot actuator 53,        and a control signal S_(CTB) for controlling the tangential        position of the pivot actuator 53,    -   to generate at its output 96, a control signal S_(L2) for        controlling the second laser beam 41.

The control circuit 90 further has a read signal input 91 for receivinga read signal S_(R) from the optical detector 35.

FIG. 1B illustrates that the optical detector 35 comprises a pluralityof detector segments, in this case four detector segments 35 a, 35 b, 35c, 35 d, capable of providing individual detector signals A, B, C, D,respectively, indicating the amount of light incident on each of thefour detector quadrants, respectively. A first line 36, separating thefirst and fourth segments 35 a and 35 d from the second and thirdsegments 35 b and 35 c, has a direction corresponding to the tangentialdirection (also called track direction). A second line separating thefirst and second segments 35 a and 35 b from the fourth and thirdsegments 35 d and 35 c, has a direction corresponding to the radialdirection. Since such four-quadrant detector is commonly known per se,it is not necessary here to give a more detailed description of itsdesign and functioning.

FIG. 1B also illustrates that the read signal input 91 of the controlcircuit 90 actually comprises four inputs 91 a, 91 b, 91 c, 91 d forreceiving said individual detector signals A, B, C, D, respectively. Thecontrol circuit 90 is designed to process said individual detectorsignals A, B, C, D, in order to derive data and control informationtherefrom, as will be clear to a person skilled in the art. Theprocessing can be done by code instructions executed by a signalprocessor.

A data signal S_(D) can be obtained by summation of all individualdetector signals A, B, C, D according to:S _(D) =A+B+C+D   (1)

Further, a first push-pull radial error signal S_(TE) _(—) _(radial) canbe obtained by summation of the signals A and D from all individualdetector segments 35 a and 35 d on one side of the first line 36,summation of the signals B and C from all individual detector segments35 b and 35 c on the other side of the first line 36, and taking thedifference of these two summations, according to:S _(TE) _(—) _(radial)=(A+D)−(B+C)   (2a)

Further, a second push-pull tangential error signal S_(TE) _(—)_(tangential) can be obtained by summation of the signals A and B fromall individual detector segments 35 a and 35 b on one side of the secondline, summation of the signals C and D from all individual detectorsegments 35 c and 35 d on the other side of the second line, and takingthe difference of these two summations, according to:S _(TE) _(—) _(tangential)=(A+B)−(C+D)   (2b)

Further, assuming that a cylindrical lens (not shown in the figure forsake of simplicity) is placed in front of the optical detector 35, afocal error signal S_(FE) can be obtained by summation of the signals Aand C from one pair of individual detector segments 35 a and 35 cdiagonally opposite to each other, summation of the signals B and D fromthe other pair of individual detector segments 35 b and 35 d diagonallyopposite to each other, and taking the difference of these twosummations, according to:S _(FE)=(A+C)−(B+D)   (3)

In order to compensate light intensity variations of the beam as awhole, these error signals can be normalised by division by the datasignal to obtain normalised error signals RES_radial/RES_tangential, andfocal error signals FES, according to:RES_radial=S _(TE) _(—) _(radial) /S _(D)   (4a)RES_tangential=S _(TE) _(—) _(tangential) /S _(D)   (4b)FES=S _(FE) /S _(D)   (5)

The above formulas are basically correct for the data beam individuallyas well as for the tilt beam individually.

FIG. 2 is a graph illustrating the normalised error signal RES_radial asa function of radial position of the lens 34, for a situation where boththe data beam 32 and the tilt beam 42 are ON.

In the case of non-zero disc tilt, the reflected light beams are subjectto deflections, the deflection of the tilt beam differing from thedeflection of the data beam. As a result, a light intensity patterncaused by the tilt beam on the detector 35 (also indicated as detectortilt spot) will be shifted with respect to a light intensity patterncaused by the data beam on the detector 35 (also indicated as detectordata spot). This translates to a DC shift (also called beamlanding) ofthe normalised error signal RES_radial, as illustrated in FIG. 2.

The horizontal axis indicates track numbers, while the vertical axisindicates signal magnitude in arbitrary units. The solid curve 61 showsthe normalised error signal RES_radial in a case without radial tilt ofthe disc 2: it can be seen that the DC level of this signal is now equalto zero. The dashed and dotted lines 62 and 63 show the same normalisederror signal RES_radial for a case where the disc 2 has positive andnegative radial tilt (depending on the sign of the tilt angle),respectively: the DC level of this signal has now been shifted to anegative and a positive value, respectively.

Due to radial disc-tilt, the data beam 32 (which is in focus) isdisplaced in the radial direction of the detector, as depicted in theupper part of FIG. 5. Since the tilt beam 42 (which is out of focus) isseverely aberrated because of lowest-order spherical aberration, itsdisplacement in the radial direction due to radial tilt is much less, asdepicted in the lower part of FIG. 5. The same goes for the situationwith tangential disc-tilt. On the contrary, radial beam-landing (BL)caused by a radial displacement of the objective lens (OL) (as wouldoccur during tracking operation) is the same for the first and secondbeams.

For a given optical disc, it can be demonstrated that the DC levelmeasured in applying only the data beam 32 is proportional to the disctilt, and that the DC level measured in applying only the tilt beam 42is also proportional to the disc tilt, with a-priori differentproportionality coefficients. As a consequence, the difference of thesetwo DC levels is also proportional to the disc tilt. Moreover, it can bedemonstrated that this difference is proportional to the differencebetween the DC level measured in applying simultaneously the data andtilt beam 32 and 42, and the DC level measured in applying only the databeam 32. As a consequence, the difference between the DC level measuredin applying simultaneously the data and tilt beam 32 and 42, and the DClevel measured in applying only the data beam 32, is proportional to thedisc tilt, either radial or tangential.

The radial and tangential disc tilt can thus be measured withoutinterrupting the data beam 32 used for read or write operations, but inswitching ON the tilt beam while the data beam 32 is ever applied on theoptical disc.

A radial tilt angle θ_radial and a tangential tilt angle θ_tangentialcan be calculated as the difference between two intermediate values:θ_radial=DC[RES_radial(T+D)]−DC[RES_radial(D)]  (6a)θ_tangential=DC[RES_tangential(T+D)]−DC[RES_tangential(D)]  (6b)wherein:

-   -   DC[x] indicates the DC level of a signal x,    -   RES_radial (D) indicates the normalised error signal RES_radial        for a case with only the data beam being switched ON,    -   RES_tangential (D) indicates the normalised error signal        RES_tangential for a case with only the data beam being switched        ON,    -   RES_radial (T+D) indicates the normalised error signal        RES_radial for a case with both the data beam and the tilt beam        being switched ON,    -   RES_tangential (T+D) indicates the normalised error signal        RES_tangential for a case with both the data beam and the tilt        beam being switched ON.

It is noted that the illustrative FIG. 2 relates to a situation of trackcrossings for a case where a tracking servo loop is open. However,during normal disc drive operation, the tracking servo loop is closed inorder to have the focal spot F stay on track, so that the radialactuator 51 is controlled to keep the normalised error signalsRES_radial and RES_tangential equal to zero. Consequently, in view ofthe shifted DC level of RES_radial and RES_tangential, the focal spot Fis actually displaced with respect to the centre of the track beingfollowed.

In the following, statement RES will stand either for RES_radial orRES_tangential indifferently for facilitating the understanding.

FIG. 3 is a timing diagram illustrating the operation of the disc drive1 in accordance with the present invention. Curve 71 represents theoperation of the data beam 32, which is constantly switched ON. Curve 72represents the operation of the tilt beam 42. Normally, the tilt beam 42is switched OFF, but at regular intervals the tilt beam 42 is brieflyswitched ON. In FIG. 3, the tilt beam is switched ON at times t1 and t3,and switched OFF at times t2 and t4.

T_(ON) indicates a time period during which the data beam 32 is switchedON and the tilt beam 42 is switched ON, having a duration τ_(ON)=t2−t1.This time period will be indicated as tilt measuring phase.

T_(OFF) indicates a time period during which the data beam 32 isswitched ON and the tilt beam 42 is switched OFF, having a durationτ_(OFF)=t3−t2. This time period will be indicated as normal phase.

Curve 73 illustrates the normalised error signal RES in a closed radialservo loop situation, while an operational mode of the control circuit90 is illustrated at 74. During the normal phase T_(OFF), the controlcircuit 90 uses the normalised error signal RES as calculated from theoptical detector 35 output signal S_(R) for controlling the radialactuator 51, as normal, indicated by S_(R) in FIG. 3.

At the times t1 and t3, the control circuit 90 stores the current valuesof the signals S_(D), RES and FES in a memory 97, after which thecontrol circuit 90 switches the second laser 41 to its ON state.

The values of the signals S_(D), RES and FES thus stored, measuredduring the normal phase T_(OFF), will be indicated as S_(D)(OFF),RES(OFF) and FES(OFF), respectively.

During the tilt measuring phase T_(ON), the control circuit 90 generatesits actuator control signals S_(CR), S_(CF), S_(CTA)/S_(CTB) for theradial actuator 51, the focus actuator 52 and the pivot actuator 53,respectively, on the basis of the values of the signals S_(D)(OFF),RES(OFF) and FES(OFF) read from memory 97, indicated by M in FIG. 3.

At the times t2 and t4, the control circuit 90 switches the second laser41 to its OFF state, after which the control circuit 90 returns tonormal operation (S_(R)).

In order to measure tilt, the control circuit 90 also measures a valueof the error signal RES from the optical detector 35 output signal S_(R)during the tilt measuring phase T_(ON). This measured value may be onesample, for instance taken at approximately time t1+0.5*τ_(ON), or anaverage of multiple samples taken during the tilt measuring phaseT_(ON). This measured value will be indicated as RES(ON). This measuredvalue RES(ON) may be processed immediately, or also stored in memory 97.

The control circuit 90 is now able to derive a tilt-indicating signalS_(TILT) indicative of the disc tilt angle θ on the basis of RES(OFF)and RES(ON). The tilt-indicating signal S_(TILT) may be calculated asthe difference between two intermediate values:S _(TILT) =RES(ON)−RES(OFF)   (7)

Alternatively, it is also possible that the control circuit 90 measuresthe value of the error signal RES shortly after termination of the tiltmeasuring phase T_(ON), this measured value being indicated asRES′(OFF). Then, the control circuit 90 is able to derive atilt-indicating signal S_(TILT) on the basis of RES′(OFF) and RES(ON).The tilt-indicating signal S_(TILT) may be calculated as:S _(TILT) =RES(ON)−RES′(OFF)   (8)

Preferably, the control circuit 90 is designed to measure RES(OFF) attime t1 (or shortly before) and to also measure RES′(OFF) at time t2 (orshortly after), to average these two measures, and to calculate thetilt-indicating signal S_(TILT) as:S _(TILT) =RES(ON)−(RES(OFF)+RES′(OFF))/2   (9)

Further, the control circuit 90 is designed to generate a control signalS_(CTA)/S_(CTB) for the pivot actuator 53 such that theradial/tangential tilt-indicating signal S_(TILT) decreases, ideallybecoming zero, indicating that the pivot actuator 53 has obtained aposition well-matched to the disc tilt. A closed loop-control can beused to this end.

Experiments have been performed with a DVD disc rotating at a frequencyof approximately 4.7 Hz. These experiments have shown that it ispossible to have a duration τ_(ON) of the tilt measuring phase T_(ON) of150 μs without affecting the data readout or the stability of the radialservo loop. Further, no significant increase in the jitter has beenobserved in these experiments. This means that the chance that theoptical pickup deviates from track during a period of 150 μs may beignored. On the other hand, this period is long enough for obtainingadequate measuring samples from the tilt beam.

Each tilt measuring phase T_(ON) corresponds, in fact, with a tiltmeasurement at substantially one location on disc. In the case of a discrotation at 4.7 Hz, a tilt measuring duration τ_(ON) of 150 μscorresponds, at a track radius of 5 cm, to a track portion ofapproximately 0.2 mm. As should be clear to a person skilled in the art,the number of tilt measurements per disc revolution can suitably be setby setting an appropriate value for the duration of the normal phaseT_(OFF). This duration T_(OFF) should preferably be selected above acertain minimum duration in order to allow the actuator system 50 toobtain a stable position. For the parameters given above, a suitablevalue for such minimum duration is approximately 1 ms.

In FIG. 3, the basic principle of the present invention is illustratedby showing that the tilt beam 42 is repeatedly switched ON and OFF,illustrated by a rectangular curve 72 which defines the tilt measuringphase T_(ON) and the normal phase T_(OFF). This curve is not to beinterpreted as meaning that the light intensity profile of the tilt beam42 should only have a rectangular shape.

On a larger scale than FIG. 3, FIG. 4A illustrates a preferred lightintensity profile of the tilt beam 42, indicated by curve 75. Thevertical axis of FIG. 4A indicates light intensity, the horizontal axisindicates time.

At times t before t1 and after t2, the light intensity I(42) is zero. Attime t1, the light intensity I(42) starts to rise, obtains a maximum ata time t0=(t1+t2)/2, and then decreases continuously to become zero attime t2. The shape of curve 75 between times t1 and t2 is a preferablycosine shape symmetrical with respect to time t0. This particular shapeprevents from spectral components in the data frequency range.

In order to provide such light intensity profile, the control circuit 90preferably comprises a shape memory 81, a digital-to-analog converter82, and a low-pass filter 83, as illustrated in FIG. 4B. The shapememory 81 contains information on the shape of the light intensityprofile to be produced, for instance in the form of a formula or alook-up table. With such a cosine shape, it is prevented that spectralcomponents in the data frequency range are introduced into the opticaldetector 35 output signal S_(R).

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, in the above it is explained that the signals S_(D), RESand FES are stored in memory and that the control circuit 90 generates,during the tilt measuring phase T_(ON), its actuator control signalsS_(CR), S_(CF), S_(CTA)/S_(CTB) on the basis of these stored values inorder to freeze the actuator positions. However, it is also possiblethat these actuator control signals S_(CR), S_(CF), S_(CTA)/S_(CTB)themselves are stored in memory, and that the control circuit 90, duringthe tilt measuring phase T_(ON), generates its actuator control signalsS_(CR), S_(CF), S_(CTA)/S_(CTB) by reading the memory and repeating thevalues read from memory. Further, if the actuators 51, 52, 53 arerelatively slow, and/or if the tilt measuring phase duration τ_(ON) isrelatively short, it is possible that the actuators do not respond tothe change in the normalised radial error signal RES illustrated bycurve 73, in which case it is not necessary to freeze the actuatorcontrol signals. However, freezing the actuator control signals providesan increased robustness of the system.

The disc drive apparatus 1 may be designed for handling only one type ofdisc, i.e. either CD or DVD for example. In that case, the data beam 32will have the wavelength mentioned for use with CD or DVD, respectively,while the tilt beam 42 is an auxiliary laser beam which may have, inprinciple, any suitable wavelength differing sufficiently from the databeam wavelength. In such an apparatus, the data beam 32 is focussedwhile the auxiliary tilt beam is out of focus and/or sphericallyaberrated.

The disc drive apparatus 1 may also correspond to a combi-drive. Thedisc drive apparatus 1 is thus designed for handling two or more typesof disc, i.e. CD as well as DVD for example. In that case, the data beam32 will have the wavelength mentioned for use with CD or DVD,respectively, while the tilt beam 42 will have the wavelength mentionedfor use with DVD or CD, respectively. In such an apparatus, the CD-typebeam is focussed when a CD is being handled, in which case the DVD-typebeam is out of focus and/or spherically aberrated. Alternatively, theDVD-type beam is focussed when a DVD is being handled, in which case theCD-type beam is out of focus and/or spherically aberrated.

1. Method of measuring the tilt of an optical disc (2) in an opticaldisc drive (1), said method comprising : a step of directing to theoptical disc during a normal phase (T_(OFF)), a first laser beam (32)having a first optical characteristic for writing/reading informationinto/from the optical disc, a step of deriving a first intermediatevalue (RES(OFF)) from a first normalized error signal obtained afterreflection of said first laser beam (32) on the optical disc, a step ofdirecting to the optical disc during a tilt-measuring phase (T_(ON)),said first laser beam (32) and a second laser beam (42) having a secondoptical characteristic, a step of deriving a second intermediate value(RES(ON)) from a second normalized error signal obtained afterreflection of said first and second laser beams (32, 42) on the opticaldisc, a calculation step of deriving a tilt-indicative signal (S_(TILT))from the difference between said second and first intermediate values.2. Method according to claim 1, wherein the first laser beam (32) has afirst wavelength and wherein the second laser beam (42) has a secondwavelength.
 3. Method according to claim 2, wherein the second laserbeam (42) has a focus point coinciding with a focus point of the firstlaser beam (32).
 4. Method according to claim 1, wherein the first laserbeam (32) has a first focus point, the second laser beam (42) has asecond focus point located at an axial distance from the first focuspoint.
 5. Method according to claim 4, wherein the first laser beam (32)and the second laser beam (42) have the same wavelength.
 6. Methodaccording to claim 1, wherein the first laser beam (32) has a firstwavelength, and the second laser beam (42) has a second wavelength, thefirst laser beam (32) has a first focus point, and the second laser beam(42) has a second focus point located at an axial distance from thefirst focus point.
 7. Method according to claim 1, wherein, in the tiltmeasuring phase (T_(ON)), the intensity of the second light beam (42) isintended to continuously rise from zero to a maximum value atapproximately half-time (t0) of the tilt measuring phase (T_(ON)), andsubsequently intended to continuously decrease from said maximum valueto zero.
 8. Method according to claim 1, wherein the first intermediatevalue (RES(OFF)) is obtained shortly before the start (t1) or shortlyafter the end (t2) of the tilt measuring phase (T_(ON)), the secondintermediate value (RES(ON)) is obtained within the tilt measuring phase(T_(ON)).
 9. Method according to claim 1, wherein the first intermediatevalue (RES(OFF)) is derived from the average of a first measure obtainedshortly before the start (t1) of the tilt measuring phase (T_(ON)), anda second measure obtained shortly after the end (t2) of the tiltmeasuring phase (T_(ON)), the second intermediate value (RES(ON)) isobtained within the tilt measuring phase (T_(ON)).
 10. Method accordingto claim 8, wherein the second intermediate value (RES(ON)) is obtainedfrom a measure obtained at a central time (t0) within the tilt measuringphase (T_(ON)).
 11. Method according to claim 1, further comprising astep of freezing, during the tilt measuring phase (T_(ON)), theactuation of at least one lens actuator of the optical disc drive (1).12. Optical disc drive (1) for writing/reading information into/from anoptical disc (2), said optical disc drive (1) comprising means formeasuring the tilt of said optical disc (2), said means comprising:first means for generating and directing to the optical disc during anormal phase (T_(OFF)), a first laser beam (32) having a first opticalcharacteristic for writing/reading information into/from the opticaldisc, calculation means (90) for deriving a first intermediate value(RES(OFF)) from a first normalized error signal obtained afterreflection of said first laser beam (32) on the optical disc, secondmeans for generating and directing to the optical disc during atilt-measuring phase (T_(ON)), said first laser beam (32) and a secondlaser beam (42) having a second optical characteristic, calculationmeans (90) for deriving a second intermediate value (RES(ON)) from asecond normalized error signal obtained after reflection of said firstand second laser beams (32, 42) on the optical disc, calculation means(90) for deriving a tilt-indicative signal (S_(TILT)) from thedifference between said second and first intermediate values. 13.Optical disc drive according to claim 12, wherein the first laser beam(32) has a first wavelength and wherein the second laser beam (42) has asecond wavelength.
 14. Optical disc drive according to claim 13, whereinthe second laser beam (42) has a focus point coinciding with a focuspoint of the first laser beam (32).
 15. Optical disc drive according toclaim 12, wherein: the first laser beam (32) has a first focus point,the second laser beam (42) has a second focus point located at an axialdistance from the first focus point.
 16. Optical disc drive according toclaim 15, wherein the first laser beam (32) and the second laser beam(42) have the same wavelength.
 17. Optical disc drive according to claim12, wherein the first laser beam (32) has a first wavelength and whereinthe second laser beam (42) has a second wavelength, the first laser beam(32) has a first focus point and wherein the second laser beam (42) hasa second focus point located at an axial distance from the first focuspoint.
 18. Optical disc drive according to claim 12, further comprising:an objective lens (34), lens actuators (51, 52, 53) for positioning theobjective lens (34), means for freezing, during the tilt measuring phase(T_(ON)), the actuation of at least one lens actuator (51, 52, 53). 19.Optical disc drive according to claim 12, intended to handle one disctype (for example CD or DVD or Blu-Ray) only, wherein the second lightgenerating device (41) is an auxiliary light source.
 20. Optical discdrive according to claim 12, intended to handle at least two differentdisc types (for example: CD, DVD, Blu-Ray), wherein: the first means forgenerating and directing are adapted to generate the first light beam(32) suitable for handling a first one of said disc types, the secondmeans for generating and directing are adapted to generate the secondlight beam (42) suitable for handling a second one of said disc types.